Return of Fishes to the Sea

Return of Fishes to the Sea
Marine bony fishes maintain the salt
concentration of their body fluids at
approximately one-third that of seawater
(body fluids = 0.3 to 0.4 gram
mole per liter [M]; seawater = 1 M).
They are hypoosmotic regulators because they maintain their body fluids
at a lower concentration (hence hypo-) than their seawater environment.
Bony fishes living in the oceans
today are descendants of earlier freshwater
bony fishes that moved back
into the sea during the Triassic period
approximately 200 million years ago.
During many millions of years that
freshwater fishes were adapting them-selves so well to their environment,
they established an ionic concentration
in the body fluid equivalent to approximately
one-third that of seawater.
The body fluid of terrestrial vertebrates
is remarkably similar to that of dilute
seawater too, a fact that is undoubtedly
related to their ancient marine
heritage.

By expressing concentration of salt in seawater
or body fluids in molarity,we are saying
that the osmotic strength is equivalent
to the molar concentration of an ideal
solute having the same osmotic strength. In
fact, seawater and animal body fluids are
not ideal solutions because they contain
electrolytes that dissociate in solution. A
1 M solution of sodium chloride (which dissociates
in solution) has a much greater
osmotic strength than a 1 M solution of glucose,
an ideal solute (nonelectrolyte) that
does not dissociate in solution. Consequently,
biologists usually express osmotic
strength of a biological solution in osmolarity
rather than molarity. A 1 osmolar solution
exerts the same osmotic pressure as a
1 M solution of a nonelectrolyte.

When some freshwater bony fishes
of the Triassic period ventured back to
the sea, they encountered a new set of
problems. Having a much lower internal
osmotic concentration than the seawater
around them, they lost water
and gained salt. Indeed a marine bony
fish literally risks drying out, much like
a desert mammal deprived of water.

To compensate for water loss a
marine fish drinks seawater (Figure
32-3). This seawater is absorbed
from the intestine, and the major sea
salt, sodium chloride, is carried by the
blood to the gills, where specialized
salt-secreting cells transport it back
into the surrounding sea. Ions remaining
in the intestinal residue, especially
magnesium, sulfate, and calcium, are
voided with the feces or excreted by
the kidney. In this indirect way, marine
fishes rid themselves of the excess
sea salts they have drunk, and replace
water lost by osmosis. Samuel Taylor
Coleridge’s ancient mariner, surrounded
by “water, water, everywhere,
nor any drop to drink” undoubtedly
would have been tormented even
more had he known of the marine
fishes’ ingenious solution for thirst. A
marine fish regulates the amount of
seawater it drinks, consuming only
enough to replace water loss and no
more.

The cartilaginous sharks and rays
(elasmobranchs) achieve osmotic balance
differently. This group is almost
totally marine. The salt composition
of shark’s blood is similar to that of
the bony fishes, but the blood also
carries a large content of organic compounds,
especially urea and trimethylamine
oxide. Urea is a metabolic
waste that most animals quickly
excrete. The shark kidney, however,
conserves urea, allowing it to accumulate
in the blood and raising the blood
osmolarity to equal or slightly exceed
that of seawater. With osmotic difference
between blood and seawater
eliminated, water balance is not a
problem for sharks and their kin; they
are in osmotic equilibrium with their
environment.

The high concentration of urea in the
blood of sharks and rays—more than 100
times as high as in mammals—could not be
tolerated by most other vertebrates. In the
latter, such high concentrations of urea disrupt
peptide bonds of proteins, altering
protein configuration. Sharks have adaptedbiochemically to the presence of urea that
permeates all their body fluids, even penetrating
freely into cells. So accommodated
are elasmobranchs to urea that their tissues
cannot function without it, and their heart
will stop beating in its absence.